Rep. John Larson (D-Conn.) was unseated Tuesday by his primary challenger, former Hartford Mayor Luke Bronin, according to the Associated Press.Why it matters: Larson is the 7th House Democrat ousted in their primary this year. Only one other election cycle in the 21st century has seen this many incumbent losses in primaries.Bronin's challenge to Larson came after the 78-year-old incumbent suffered a widely publicized freeze-up during a House floor speech that he later said was the result of a complex partial seizure.Bronin earned the support of the state Democratic Party, while Larson had the backing of House Minority Leader Hakeem Jeffries (D-N.Y.) and the rest of Connecticut's congressional delegation.State of play: Bronin, 47, proved to be a prodigious fundraiser, bringing in $3.1 million as of July 22 to Larson's $2.7 million over the same period, according to Federal Election Commission filings.Bronin also had the most support from an outside backer: VoteVets, a group that works to elect Democratic military veterans, said in FEC filings it spent over $1 on ads boosting the retired Navy intelligence officer.Larson's top outside spender was Defend the Vote, a Democratic group whose stated mission is to elect a "pro-democracy" majority in Congress. They spent just over $500,000.Between the lines: Bronin ran a campaign that focused heavily on the need for generational change in Congress and the Democratic Party."John Larson is a good man, but he's been in Congress since the 90s. The stakes are just too high to keep doing the same thing," Bronin said in one ad.Both men also tried to burnish their progressive credentials at a time when Democrats are increasingly favoring progressives in safe-seat primaries.The progressive Working Families Party, which spent about $50,000 supporting Larson, ran an ad labeling Bronin a "corporate Democrat" who was "using us for political ambition."Zoom out: Larson is part of a growing cohort of older Democrats who have lost to significantly younger primary opponents this year, including Reps. Al Green (D-Texas), Diana DeGette (D-Colo.), Adriano Espaillat (D-N.Y.) and Shri Thanedar (D-Mich.).While DeGette, Espaillat and Thanedar all lost to challengers who ran to their left, Green and Larson both positioned themselves as stalwart progressives against opponents who focused on their age.Reps. Doris Matsui (D-Calif.) and Mike Thompson (D-Calif.) both face November run-offs with younger Democratic rivals.Other members who are 70 or older, including Reps. Ed Case (D-Hawaii) and Brad Sherman (D-Calif.), were able to fend off their younger primary opponents.